Guys, just a warning, the GS-X MK2 amp has less than 1 ohm output impedance. It's not good for the HD800 according to Skeptic because it's not 40ohm output impedance and not made by Sennheiser. Stay away(well atleast Skeptic should stay alway because his magic number is 40ohms).
Your reading comprehension really leaves something to be desired...please stop misconstruing and mischaracterizing me as some sort of absolutist.
I only suggested the anax mod or a higher ohm amp because of your particular comments about treble sensitivity, which many of us have dealt with to greater or lesser degrees. In fact, the same criticisms you have raised are exactly what lead anax, arnaud, purrin etc to work up and create various iterations of the mod that you have refused to consider. For your further reference, in addition to commenting on the impact of amp output impedance, anax and purrin have both stated that they do not care for the gsx mk2 with hd800s, even though it is a very popular pairing among some on HF. (Really wish the mods would relax the rules on linking certain other forums so you could read this all first hand.)
I haven't had the pleasure of hearing a gsx, but I do have dynahi boards and all of the discontinued transistors sitting on my bench - so maybe one day I will be able to weigh in on this to a greater degree. I have the utmost respect for Dr Gilmore's time tested designs, but my sense is still that dynalo derivatives may not be the ideal hd800 amp for those with high treble sensitivities. (As you may or may not be aware, dynalos were not designed with high impedance phones in mind.)
On the flip side, 40 ohms isn't some magic number (as far as I know). It is just what Senn chose for its own amp, which a lot of people seem to like on the SS side of the aisle. I have no personal experience with this product either (sorry Priidik!), but it stands to reason that its output impedance is in the range that the manufacturer thought was commendable from the standpoint of electrical damping.
I run my mainline with the output impedance set to 32 ohms, and on occasion bump it to 64 ohms when I feel like a change and a little more midbass. Bass response with hd800s is noticeably tighter and more linear, and generally preferable to me, at 32 ohms, although I'm sure opinions vary on this point. I can also tell you that I prefer my hotrodded crack (120 ohms) over my true 0 ohm agdr boosted / lme49990 modded o2 (http://www.agdraudio.com/Page2.html), which is the leanest and most treble happy of my amps with hd800s. I love it with my ADs though.
In any event, hope this clears a few matters up for you in terms of where I've been coming from. If you thought your hd800s sounded full, pleasing and accurate in terms of timbre (and not fatiguing) with a 0 ohm amp, then that would be the amp to stick with. Obviously that isn't the case. Time to man up and address the issues or cut your losses and quit the QQ'ing.